1. Field of the Invention
The present invention generally relates to a reflective screen and a display system that display an image projected by a projection device, and particularly relates to a reflective screen and a display system that allow projection of an image from close range.
2. Description of the Related Art
There have been projectors configured to enlarge and project an image using a CRT or liquid crystal and overhead projectors (OHPs) configured to enlarge and project an image formed on a transparent sheet. These projectors usually project an enlarged image onto a screen. It is preferable that such a projector be placed in front of the screen, but the projector is sometimes not located in front of the screen because of limitations such as viewers' positions.
If, for example, a projector is disposed under the screen, the incident angle of the projection light on the screen varies in the vertical direction of the screen, so that the luminance decreases from the lower side to the upper side.
In view of such a problem, Japanese Patent Laid-Open Publication No. 2005-134448 (Patent Document 1) discloses a screen including a Fresnel reflective surface. FIG. 17A is a cut-away side view illustrating a screen 50 disclosed in Patent Document 1. In this screen 50, a reflective layer 53 includes multiple small projections and recesses such as, for example, arcuate reflective element surfaces 51a each having about 0.1 mm width. The reflective element surfaces 51a are arranged in a concentric pattern to form a circular Fresnel shape. The circular Fresnel reflective element surfaces 51a reflect an incident light 11a from a projector as collimated beams in the direction perpendicular to the front surface of the screen 50, thereby preventing luminance from decreasing toward the upper side.
Japanese Patent Laid-Open Publication No. 2004-177427 (Patent Document 2) discloses a screen including tilted surfaces as reflective surfaces. FIG. 17B is a cut-away side view of the screen disclosed in Patent Document 2. This screen includes tilted surfaces 61 as reflective surfaces, which are configured to reflect light beams projected from a projector in a desired direction and are alternately arranged with light absorbing surfaces 62, thereby preventing reduction of luminance in the upper side of the screen.
Japanese Patent Laid-Open Publication No. 2006-215162 (Patent Document 3) discloses a screen including projecting portions with reflective surfaces formed thereon. FIG. 17C is a cut-away side view of the screen disclosed in Patent Document 3. This screen includes a large number of projecting portions 71 of the same shape, which are regularly and two-dimensionally arranged. A reflective surface is formed on a part of the surface of each projecting portion facing the incident direction of projection light. The remaining part of the surface of the projecting portion is covered with a coating having high light absorption, thereby preventing the outside light from being reflected to the viewer side. With this configuration, the screen achieves high contrast and high luminance.
There is a tendency to place a projector closer to screens because of limited space for the projector. FIG. 18 shows an example of luminance variation in a related-art reflective screen with respect to the incident angle of projection light on the screen. Luminances in predetermined positions on the screen were measured by an illuminance meter placed parallel to the screen surface. The luminances are represented by relative values to the luminance at a 20-degree incident angle (with reference to the line perpendicular to the screen surface). The luminance at a 20-degree incident angle was defined as 1. As shown in FIG. 18, in the related-art reflective screen, the substantial brightness decreases as the incident angle of the light from the projector on the screen increases.
For example, the substantial amount of light at a 50 degree incident angle is 50% of that at an about 20 degree incident angle. That is, in the related-art screen, the ambient luminance (substantial brightness) is reduced according to the incident angle. The shorter the distance from the regular projection position of the projector to the screen is, which regular projection position is determined based on the focal distance of the projector, the more prominent the luminance reduction due to a change in the distance from the regular projection position. If the regular projection position of a projector is relatively far from the screen, because a diffuse surface of the screen diffuses light, a change in the distance to the screen does not result in such a sharp drop in luminance.
That is, the closer the projection position is, the more the screen for the projector is required to reduce luminance variation and luminance decline by compensating for the change in the distance due to limitations on space.
Patent Document 1-3 do not take into consideration the problem with a projector being located close to the screen. For example, the screens of Patent Document 1 and 2 cannot react to a change in the position of a projector because of a Fresnel focal position being constant and or the tilted surfaces being uniform. More specifically, the screen of Patent Document 1 requires a projector to be located in the Fresnel focal position. If the projector is not in the Fresnel focal position, a desired reflection angle cannot be achieved due to the fixed angle of the Fresnel reflective surface, resulting in a brightness variation within the screen. As for the screen of Patent Document 2, because the tilted surfaces 61 are tilted uniformly, only a small number of the tilted surfaces 61 can reflect light to the viewer side depending on the irradiation position of a projector. As for the screen of Patent Document 3, because the projecting portions 71 reflect projection light in different directions, some light is reflected to the viewer side even if the irradiation position of a projector is changed. However, if the projector is located within a short distance, the absolute luminance becomes insufficient.